A case study of the effect of grain size on the oxygen permeation flux of BSCF disk-shaped membrane fabricated by thermoplastic processing
Highlights
► Planar BSCF membranes by thermoplastic pressing and sintering. ► Oxygen permeation fluxes were not affected by variation of grain size. ► Random grain orientation due to the processing. ► Grains and grain boundaries electronic conductivity are similar.
Introduction
Perovskite-based, mixed ionic–electronic conductors (MIEC) are of great interest because of their 100% oxygen permselectivity at elevated temperatures. These materials are widely under development as ceramic membranes for oxygen separation and in catalytic partial oxidation reactors, as electrodes for solid oxide fuel cells and for high temperature electrolysis [1], [2], [3], [4]. Oxygen permeation in MIEC membranes is driven solely by a difference of oxygen partial pressure (oxygen potential gradient) on either side of the membranes.
Since Teraoka reported high oxygen permeation flux in La1−xSrxCo1−yFeyO3−δ (LSCF) ceramic membranes in the late 1980s [5], [6], a large number of studies have been carried out to optimize oxygen permeation in this system, with SrCo0.8Fe0.2O3−δ (SCF) showing significant promise. However, later studies revealed that the SCF phase was not stable below 800 °C at oxygen partial pressures lower than about 0.1 atm due to oxygen vacancy ordering [5], [7], [8], [9], [10].
It was later found that the structural and chemical stability of the SCF membranes were greatly improved by partial substitution of Sr with Ba, retaining a high oxygen permeation flux for compositions around Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF). Recently, BSCF has received increasing attention for use in ceramic oxygen membranes [11], [12], [13], [14], [15].
It has been reported that the oxygen permeation flux of the MIEC membranes is sensitive to microstructural features, such as grain size and grain boundary structure [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. The microstructure depends both on processing history, and especially the sintering procedure, even for similar components. The preparation of MIEC membranes can be considered in terms of three steps: (1) powder synthesis, (2) shaping, and (3) sintering. The influence of each of these steps on the microstructure and oxygen permeation flux of the MIEC membranes has been investigated over the last decade, with several different behaviors reported [13], [16], [17], [19], [20], [21], [24].
In the case of BSCF, previous studies have resulted in conflicting data, especially with regard to the influence of grain size on permeation [13], [14], [18], [22], [23], [24], [25], [26]. The effect of the microstructural on the oxygen permeation flux cannot be generalized and the contradictions could be attributed to microstructural variations caused by differences in powder synthesis routes and sintering conditions used by different groups.
In this study, we have examined the effect of grain size on the oxygen permeation flux of Ba0.5Sr0.5Co0.8Fe0.2O3−δ membranes, fabricated by warm pressing. Three samples with differing grain sizes were prepared by sintering at different temperatures. The associated microstructural changes were characterized using scanning electron microscopy (SEM) combined with electron backscatter diffraction (EBSD) for texture analysis and conductive mode (CM) microscopy to observe the local electrical behavior in the BSCF membrane. Oxygen permeation experiments were also carried out on the membranes sintered at each temperature.
Section snippets
Sample preparation
Commercially available Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) powder (Treibacher, Austria), with a mean particle size of 3 μm and a specific surface area of 1.55 m2/g was used. Mean particle size (dBET) also was calculated from the BET according to dBET = 6/[ρthSBET], where ρth is the theoretical density of the BSCF powder measured by He-pycnometer and SBET is the measured surface area by BET and a value of 0.70 μm was determined.
Fig. 1 shows the thermoplastic processing route used for fabricating the disk
Results
Table 1 gives the sintered density, average grain size and porosity of the BSCF membranes sintered at different temperatures. The sintered densities and grain size both increase with increasing sintering temperature.
Fig. 4 shows Secondary Electron (SE) images of the membrane samples sintered at each temperature, with superimposed EBSD orientation maps, in which each grain's crystallographic orientation relative to the image normal is given by the color in the unit triangle. (For interpretation
Discussion
The EBSD data show that all three samples have close to random textures with similar grain boundary structure distributions. The only microstructural parameters that change are grain size and porosity: porosity only slightly. Within the accuracy of our experiment, changes in grain size in the range 3–18 μm did not affect the oxygen flux at temperatures in the range 800–1000 °C.
Wang et al. [13] reported a positive, albeit weak, correlation between flux and grain size, in the range 30–140 μm, which
Conclusion
BSCF membranes, 0.6 mm in thickness, were fabricated by the warm pressing method using thermoplastic processing. Different grain sizes were obtained by sintering the samples at 1000 °C, 1050 °C and 1100 °C for 2 h. A density of the 96% was achieved for the membrane sintered at 1000 °C and 98% for 1100 °C. An average grain size of 3.23 μm for fine grained membrane and 18.25 μm for coarse grained membrane were determined. Permeation flux measurements showed no difference in performance for membranes
Acknowledgements
The authors would like to acknowledge the financial contribution provided by Swiss Electric Research and the Competence Center for Energy & Mobility (CCEM) in Switzerland. The authors also would like to thank Mr. Christoph Neururer at Université de Fribourg for the EBSD measurements.
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2017, Chinese Journal of Chemical EngineeringCitation Excerpt :Perovskite-type (normally in the form of ABO3) [9–13] MIEC membranes always present relatively higher oxygen permeation fluxes among various MIEC membranes due to their higher ambi-polar conductivity for ionic and electronic. So far, MIEC membranes in different geometries such as disk, tubular and hollow-fiber [14–18] have been widely studied. Modeling has become a useful tool for oxygen permeation process simulation and theoretical analysis for perovskite oxides [19–21].